1. Field of the Invention
The present invention relates to the bit of a boring apparatus used for drilling oil wells, a geothermal wells or the like and, more particularly, to a drill bit which is fixed to the leading end of a drill pipe and which includes roller cutters attached to inclined pins, the drill bit boring a hole into the ground as the drill pipe is turned.
2. Description of the Prior Art
FIGS. 6 and 7 show a prior art boring drill bit which has roller cuutters, the drill bit including a shank 1, which is threaded to be connected to a drill pipe, and two or three legs 2 which are integral with the shank 1. Each leg 2 is formed with an inclined pin 3 which is inclined into the ground with respect to the center line of the shank 1 and made to project radially. The inclined pin 3 is crowned with a roller cutter 4 which has an identical conical contour and is made rotatable close to an adjacent one. FIG. 7 is a sectional view showing one of the inclined pins 3 according to the prior art. The inclined pin 3 protrudes from the corresponding leg 2 and towards the ground is formed as a straight cylinder having an equal diameter and has its ground-side end face 5 extending normal to the axis thereof. The end face 5 is formed at its center with a pilot pin 6 which has a smaller diameter. The straightly cylindrical inclined pin 3 extending from the leg 2 is formed in its outer circumference near the end face 5 with a groove 8 for fitting a plurality of bearing balls 7 therein.
From the inclined pin 3 to the leg 2, there extends a small hole 9 for feeding a lubricant to the bearing portion. The inclined pin 3 is formed with a hole 10 which is opened in the outer face of the leg 2 for guiding the balls 7 into the groove 8. The hole 10 is made to communicate with the small hole 9 so that it provides a lubricant passage. The hole 10 thus made is plugged at 11, while leaving its lubricant passage portion, so that it may prevent the balls 7 from coming out, and is welded at 12 to the leg 2.
The roller cutter 4 has its opening fitted on the outer circumference of the inclined pin 3 through a plain bearing 13 and the balls 7. As a result, the roller cutter 4 is made rotatable around the inclined pin 3. In the surface of the roller cutter 4, there are buried a number of edges 14 which are made of a tungsten carbide alloy. A flexible seal ring 15 is fitted on the root of the inclined pin 3 extending from the leg 2 so that the lubricant fed to the groove 3 will not flow out through the plain bearing 13.
When passed into the ground with the turning drill pipe, the roller cutter 4 revolves around the inclined pin 3 so that the ground is bored by the revolution of the roller cutter 4 on its axis and around the drill pipe.
The balls 7 play the roll of plain bearings and function to prevent the roller cutter 4 from coming out from the inclined pin 3.
When the oil or geothermal well is to be bored, the bit is driven into the ground as the drill pipe is turned. When either the edges of the bit or the bearings of the roller cutters are worn, the drill pipe is pulled out from the bore for replacement of the leading end portion of the drill pipe by a new one, and the drill pipe having its lower end equipped with the new bit is inserted into the bore so that the boring operation may be restarted. If the depth of the bored well is increased, it frequently takes a long time, e.g., one day to replace the bit.
For saving the boring cost, it is important to minimize the frequency of the bit replacement, i.e., to prolong the lifetime of the bit. The shortening of the bit lifetime is almost always caused by the fact that the edges are worn so much that they cannot be used any more or that the roller cutter bearings are worn so much that they cannot stand further use. The factors for determining the lifetime of the bearings are dependent upon how large the total surface area of the bearings or the projection area of the bearings upon a plane normal to the center line of the drill pipe is. Therefore, the lifetime of the bearings can be prolonged by reducing the load upon a unit area or by reducing the relative frictional speed of the bearings.